The parallel is there, like it or not. The only reason I mentioned it is to illustrate that the audio industry is behaving like any and all other industries.

I have yet to see a car made by anybody which will stand up to its published consumption levels, just as I have yet to be convinced that an amp with a THD of 0,0001% by default sounds better than another amp with a THD of 3%. Tube amps do 3% THD easy, yet many of them sound good and often better than their SS counterparts with a THD of 0,0001%.

Calculating backwards as you said is risky because you have to have a universal platform. Make a boo-boo in defining that platform and you have it wrong across the board. Declare that static THD and IM define the sound of an amplifier and you just made that boo-boo.

The auto industry will always promote a system which makes them look good, just as the audio industry will promote measuremets which make them look good, even at the expense of veracity. In fact, while I cannot prove it, I have an idea that the audio industry is what at least slows down, if not stopping altogether, the development of new measuring methods, which could be more relevant to the sound we hear in the end, just as the auto industry is surpressing alternate fuels which may require significant investment on their part.

The Top Dog is always the mass produced series production and its economics. Examples proving this are gallore.

When the Philips 721 CD player appeared 15 years ago, it was slated as a model to be sold in supermarkets (literally). The nmagazines got hold of it and lo and behold, declared it to be able to box way above its price class (true, in my view, I still have one somewhere). Then the DIY gang discovered that if you swapped its plain vanilla output op amp with a say OP 275, the sound got audibly better still. This begs the question why didn't Philips do the same?

"Complex marketing reasons" was the answer I got from Philips (I had excellent ties with their SE Europe HQ in Vienna). The cost difference would have been around $0.5 per unit, but that works out to $200,000 per a series of 400,000. What do you think, what would most manufacturers rather do, improve their product or invest that money into marketing?

Also, they would end up with a player which sounds better than the models above it, sort shooting themselves in the leg.

OK, why not use tha OP275 or some suc across the board? Ah, but that would make Philips compete with Marantz (at the time still in Philips majority ownership).

If the economic reasons can do that, imagine what they can do in promoting such measutement techniques which will show up their work, rather than show up possible faults or shortcomings.

What I see is that any efforts to really improve our measuring techniques are generally made on the fringes of the general audio industry, in small companies really trying to do better. The big companies are not interested, or if they arem, then it's strictly for in-house use.

@fas42 - you get me thinking...
I sometimes think back to the stuff we had back in the 1950's-'60's - like Quad II, Williamsons, Mullard 5-20 and 5-10, Quad 303, etc. They would have had (guessing) distortion figures of 0.05-0.1%, frequency response 30c/s-20kc/s +/- 0.5dB. Those amps sounded, and still sound, very good. 40 years ago I heard heard Quad IIs playing into ELSs in a small concert hall, and I thought then "this is as good as it needs to get". A fellow-student was given a 303, Thorens, and decent loudspeakers by Quad, which ended up being looked after by me for 6 months, and I felt the same about them.

So, I think, did the science of audio reproduction get as good back then as it needs to be for beings with a limit to their hearing (Iknow damned well that I can't hear 20kc/s, or distortion below .05%). There may be people out there who can hear any distortion above 0.0005%, but they must lead a tortured existence, and I've never knowingly met one.
Not decrying the never-ending striving for technical improvement, but it has perhaps become just a fanatical hobby - like stamp-collecting. Both are fine, providing you don't confuse them with living.
Just saying....!

That is something I would toss into the simulator to check. It looks almost like a flyback power supply. There is already capacitance from the screen to the cathode to hold the charge. And the sudden change in current through the transformer is not good if linearity is on your roadmap. It may add some nice harmonics to "tune up" your tone.

This was reference kick-back diode . On measurements it did nothing obvious . My dear friend Juan in Argentina who is the local TV repairman with the PHD ( I suspect from his references to Maxwell when problem solving ) said it is analogous to a voltage multiplier . I suspect if so a PP EL 34 amplifier with distributed load must be capable of doing the same ? My brother always said double voltage factors for PP operation . Is it safe to run an EL34 in SE at 450 V with a diode connected forward biased to g2 .

If my spectrum analyzer is right it is harmless . If the author is right that it deals with complex loads better it is a very cheap modification .

My brother said UL s not simple negative feedback . This diode might challenge that belief of his . He said he could see both positive and negative happening . He felt it bootstrapped or had negative feedback depending on the moment in time . For someone not given to conjecture this was a one off . He might be right and this forces it to be negative only . As you say the charge inside the device might commutate where needed when the diode is working ( reverse biased ) . What a nasty thought .

Adding a diode in the g2 connection might have some effect with a genuine tetrode, which can have negative screen current under certain conditions. It should have no effect for a pentode or a beam tetrode, as secondary electrons are kept in their place. There seems to be a modern fad of randomly sprinkling diodes into circuits where they will always be forward biased so they either do nothing or add a little more distortion.

UL is not exactly the same as ordinary NFB, because the feedback is applied to a different electrode (g2) from the input signal (g1) with slightly different curves, but it is sufficiently close to NFB for many purposes.

Would it be possible that a diode to g2 would help a situation where the back EMF of a speaker is producing different distortion compared with a resistive load ? I can contrive something to test this ? Assume not always using loop feedback . Very interesting about pure tetrodes . I have never seen one I think ? Maybe in a radio or TV front end ?

As for the voltages I will just have to rig up a test . I would have thought if the voltage was rising the measurements should change ?

I have seen signal applied to g2 to better approximate a proper triode . Would be interesting to speculate about what could be applied to g1 if so . I think EL34 even has g3 separate ? If so signal to g3 and what of the rest ? The EL34 is so close to a perfect concept . All it set out to do was be practical suspect ? It is even economical with heater current .

Some used the cathode for a UL type correction . Hafler and Quad had a small dispute over it . I suspect the Quad is best described as cathode feedback as we have always known it . It is possible to connect the secondary of a conventional transformer to the cathode to do that , it doesn't always need a third winding . If Quad are right it means a true triode can be UL connected . It is said by some that both Quad and Hafler invented UL , Hafler is to g2 . Can someone name a present production indirectly heated power triode ? Was there ever one ?

I was told by a quite famous valve man that he used the Quad method as it halved the distortion for a given gain . He was so vague as to convince me he was wrong . He might have a point . I certainly didn't find that when cloning the PYE Mozart . I can see between triode and pentode there are many possibilities and perhaps it does work better ? I recently found that 82% UL and EL 34 g2 triode to be on measurements close to identical except gain . I reason that is because EL 34 g2 triode strapped is not a true triode ? Both look excellent on the scope , pentode is very different .

10W of heater is not what I call economical. Beam tetrodes often do better in this respect.

Nothing I tried seemed to better it all things considered . I have no great interest in PP feedback valve amps . Doubtless a different world .

I have to be honest I prefer transistor amplifiers over most PP valve designs . It is very possible that the ones I tried were not very good ( TVA ) . I like the Dynaco very much , valves has nothing much to do with it . Did anyone use KT 66's or the like in one ? The Marantz 9 is very good . Both are EL 34 designs . Quad is not my favorite . I love the Quad 303 .